HE OXIDATION OF PYRITES AS A FACTOR IN 
‘THE SPONTANEOUS COMBUSTION OF COAL 


BY 


SHEO-HEN LI 


B.S., University of Illinois, 1922 
M.S., University of Illinois, 1923 


ABSTRACT OF A THESIS 


SUBMITTED IN PARTIAL FULLFILLMENT OF THE REQUIREMENTS 
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN 
CHEMISTRY IN THE GRADUATE SCHOOL OF 
THE UNIVERSITY OF ILLINOIS, 1925 


Reprinted from Industrial and Engineering Chemistry, Vol. 18, No. 12, page 1299. 
December, 1926. 


ACKNOWLEDGMENT 


This investigation was carried out under the 
direction of Professor S. W. Parr during the years 
1923-1925 in the chemical laboratory of the Univer- 
sity of Illinois. The writer takes this opportunity 
to express his most sincere appreciation and thanks 
to Professor Parr for the valuable help and direction 
which he has given. 


The writer also wishes to thank Mr. F. B. Hobart 
for his suggestions and help in carrying out this 
investigation. 


REPRINTED FROM 


Published by the Ameriean Chemical Society 


Vol. 18, No. 12 Page 1299 December, 1926 


The Oxidation of Pyrites 
as a Factor in the Spon- 
taneous Combus- 
tion of Coal’ 


By S. H. Li with S. W. Parr 
UNIVERSITY OF ILLINOIS, URBANA, ILL. 


Recent studies on the oxidation of coal seem to prove 
conclusively that oxidation proceeds very rapidly after 
a. temperature of 70° or 80° C. has been attained, and 
quickly reaches the autogenous stage, while at normal 
temperatures the oxidation is not ordinarily of suffi- 
cient magnitude to generate heat. The question there- 
fore arises as to the source and causes for an initial rise 
in temperature which is responsible for advancing the 
mass to the danger point. 

It is well known that coals that are very low in sulfur 
content are subject to this initial heating, and it is 
altogether probable that sulfur is not always the source 
of difficulty. Under certain conditions sulfur in the 
pyritic form may set up oxidation processes which may 
account for this initial heating. The present studies pro- 
ceed along the line of pyritic sulfur activity from the 
standpoint of fineness of division, form of crystallization, 
whether pyrite or marcasite, and the effect of bacterial 
and catalytic agents which might possibly be present in 
the original substance. The most pronounced indica- 
tion of oxidation was secured in the case of coals which 
started out with a high percentage of textural moisture 
which was allowed to be replaced by oxygen, and the most 
active condition at normal temperatures was secured 
with mixtures of certain types of clay. 


1 Presented before the Division of Gas and Fuel Chemistry at the 
71st Meeting of the American Chemical Society, Tulsa, Okla., April 5 to 9, 
1926. : 
(1) 


HE subject of spontaneous combustion of coal has been 

a topic of much discussion and experimentation. Owing 

to the complicated nature of the coal substance and 
the many factors that enter into the heat-producing reactions, 
no definite conclusion has been reached, especially with 
respect to the source of initial heating. Ample evidence is 
available to show that coal does heat in the presence of 
oxygen at elevated temperatures, say 80° C. or higher; yet 
this does not cover the fundamentals of the problem. It is 
unquestionable that coal piles will occasionally take fire in 
the absence of extraneous sources of heat and the question 
arises as to the initial causes for starting the heat increments. 
Presumably, the initial heat must be associated with the 
oxidation of either the carbonaceous materials or with the 
sulfur content of coal. 

Some coals with very low sulfur content may ignite, yet 
this does not mean that the pyrites is not a factor in other 
cases. Previous investigations?*? have shown that any 
form of pyrites, when finely ground, will absorb oxygen and 
enter into heat-producing reactions. Furthermore, owing to 
its low specific heat, pyrites will heat up faster than coal and 
it may act as booster for further oxidation activities. It is 
the primary purpose of this investigation to determine the 
role of pyrites in the initial heating of coal. Chief attention 
is given to the conditions which promote its oxidation at 
ordinary temperatures. 


Experimental Procedure 


The oxidation of mineral pyrites or the sulfur in. coal was 
determined by measuring the amount of increase of sulfate 
content before and after the oxidation. It was found from 
preliminary experiments that when a coal and pyrite mixture 
was oxidized in a stream of oxygen saturated with moisture 
the products of oxidation were retained in the mixture as 
sulfates regardless of the intermediate steps. There was no 
loss of sulfur due to volatile gases, although when the mineral 
pyrites was oxidized alone at 100° C. in the same way a 
small proportion of sulfur dioxide gas passed over. Thus the 
increase of sulfate content in coal-pyrite mixture is an accu- 
rate measure of the degree of oxidation of the pyrites. 

The oxidation of pyrite-coal mixture was carried out in 
an apparatus devised by Prof. J. White,* of Rose Polytechnic 
Institute. The principle and operation are self-explanatory 
from the sketch as shown in Figure 1. Its essential feature 
is a double-walled tube with ends closed with corks. The 
space between the two tubes is filled with water vapor gener- 
ated from a flask on a hot plate. The upper part of the ap- 
paratus is provided with a return condenser. The con- 
densed water returns to the flask through a by-pass. 

2 Winmill, Trans. Inst. Min. Eng. (London), 51, 500 (1916). 


3 Graham, Jbid., 6%, Pt. 2, 100 (1924). 
4 J, Franklin Inst., 178, 201 (1911). 


(2) 


In this apparatus the substance to be oxidized can be 
kept at any desired temperature for a long period of time by 
using different liquid heating media. Oxidation was carried 
out at two different temperatures, one at 100° C., using 
water as a heating medium, and the other at room tempera- 
ture, which varied from 23° to 27° C., designated as 25° C. 
At room temperature a single glass tube was used instead of 
the double-walled heating apparatus, other arrangements 
being the same. 


3 


| 


oxygen 
—_—> 


Figure 1 


Samples of 5 grams each of coal or coal-pyrite mixture, 
accurately weighed out, were put in small aluminum boats 
and placed in the inner tube. Oxygen, saturated with mois- 
ture by bubbling through a wash bottle containing distilled 
water, was passed over the coal and discharged into the 
atmosphere, since the oxidation of the coal substance is dis- 
regarded. The wash bottle containing distilled water, in 
the case at 100 ° C., was placed near the hot plate to keep 
it fairly warm and thus enable the oxygen to carry plenty 
of moisture when it enters the apparatus. After a certain 
time of oxidation duplicate samples of 5 grams each were 
transferred to large beakers and their sulfate content de- 
termined. The percentage was calculated on the basis of 
total sulfur, deducting the sulfate originally present. 


(3) 


The method used for the determination of sulfate was that 
worked out by Powell and Parr.® The method consists of 
extracting the coal with 3 per cent hydrochloric acid at 
60° C. for 40 hours. After filtration the sulfate is deter- 
mined as usual. Duplicate samples were run in all the cases 
and a good check, variable within 0.01 per cent, was fre- 
quently obtained. 


Oxidation of Sulfur Originally Present in Coal 


Four samples of Illinois coal from four counties in Illinois— 
(1) Jackson, (2) Randolph, (3) Franklin, and (4) Vermilion— 
were examined with regard to their sulfur content and rate 
of oxidation. The coals were in each case ground to pass 
through an 80-mesh sieve and oxidized in a stream of satu- 
rated oxygen as described in the experimental procedure. 
The first two—from Jackson and Randolph counties—were 
air-dried while the other two retained their textural moisture. 


Oxidized 


a 


Per Cent of Su/ohur 


Stim e ve peers 


Figure 2—Oxidation of Sulfur in Illinois Coals at 25° C. in 
Oxygen Saturated with Moisture 


The results as represented in Figures 2 and 3 show that (1) 
the rate of oxidation of sulfur in air-dry coal varies with the 
temperature; (2) coals with high initial moisture may have 
a high rate of oxidation of sulfur at normal temperatures. 
From these results it seems desirable to study the factors 
which influence the rate of oxidation. 

The difference in the rate of oxidation of the sulfur in coal 
may be due to several causes, as (1) the presence of certain 
readily oxidizable forms of pyrite in some of the samples (to 
be discussed in a later paragraph); (2) the initial textural 
moisture content in the samples; and (3) some samples con- 
tain more of the very finely divided pyrites. To prove the | 
last point a sizing test was made by sieving the samples 

5 University of Illinois, Eng. Exp. Sta., Bull. 111 (1919). 


(4) 


4 


= 


through screens of different sizes with sulfur content de- 
termined on each portion. The results of this test, together 
with related factors, are given in Tables I and II. 


Table I—Distribution of Total Sulfur with Reference to Fineness of 


Division 
Through 80 Through 140 ‘Through 200 325 mesh to 
on 140 mesh on 200 mesh on 325 mesh dust size 
Per cent Per cent Per cent Per cent 
Vermilion 5.09 24.20 26.86 43.85 
Franklin 12.59 25.09 32.85 29.47 
Jackson 9.49 19.55 21.80 49.16 
Randolph 8.44 20k 26.10 36.15 


Table II—Proximate Analysis of Samples of Illinois Coal, with 
Special Reference to Their Sulfur Content and Its Distribution in 
Various Forms 


(Organic sulfur equals the total sulfur minus sulfate sulfur plus pyritic sulfur) 


Vermilion Franklin Jackson Randolph 

Per cent “Per cent Per cent Per cent 
Moisture 10.54 6.95 1.67 3.87 
Ash 12.26 5.05 8.66 13.43 
Total sulfur 3.748 1.384 Seivs 2.590 
Sulfate sulfur 0.071 0.022 0.132 0.012 
Pyritic sulfur 1.914 0.412 3.165 0.596 
Organic sulfur 1.763 0.950 1.876 1.982 
Total oxidizable sulfur 3.677 1.362 5.041 wpagay ts 


r Oxidized 


v 


pedi ere bale aly 
Fane aa 
fepeme lm ale ala] | 


Per Cent of Sulphy 


yay oa 

SV aaa eee 
ws cod a ci 
VAS ic oa aed etd Ds i ER 


ae in Weeks 


Figure 3—Oxidation of Sulfur in Illinois Coals at 100° C. in 
Oxygen Saturated with Moisture 


The fineness factor enables us to explain the behavior of 
sulfur only ina general way. Vermilion and Jackson County 
coals have a high percentage of sulfur in the fine sizes, yet 
the latter did not show nearly so high a rate of oxidation as 
the former. The difference may be explained by the fact 
that Jackson County coal contains a considerable amount of 
sulfate sulfur which probably is very finely divided, leaving 
the actual oxidizable sulfur in the coarser portions. 


(5) 


The sulfur in Jackson and Vermilion County coals have 
about equal rates of oxidation at 100° C.; yet the latter is 
considered much more in danger of combustion in storage 
than the former. The Vermilion coal contains a high per- 
centage of sulfur, which, most important of all, has a high 
rate of oxidation at ordinary temperatures. Vermilion coal 
also seems to-be more fragile than the other samples examined. 
This would increase its danger of firing in storage. 


Pyrite 


— 


Marcasite 
Figure 4—Pyrite and Marcasite Used in This Investigation 


Oxidation of Pyrite and Marcasite 


It is often suggested in the literature that the marcasitic 
form of iron sulfide is much more readily oxidized than the 
pyritic, yet there is no experimental evidence as to what 
extent this is true. Graham,' in his study on the absorption 
of oxygen by different forms of pyrite, found that marcasite 
did not have nearly so high a rate as one sample of massive 
pyrites. He interprets his result as due to size. He said 
that ‘‘the difference observed between marcasite and the 
Cornwell pyrite may possibly be due to size, since micro- 
scopic examination of the two powders rather indicated that 
the latter, when crushed to pass through a 200-mesh sieve, 
formed finer particles than the former.” 


(6) 


In view of this uncertainty it seemed advisable to carry 
out more work along this line. One sample of cubic pyrite 
and one of marcasite were examined for this purpose (Figure 
4). The sample of pyrite, obtained from Leadville, Colo., 
came in as perfect cubes with bright, brassy surfaces evi- 
dently untouched by oxidation. The marcasite was ob- 
tained from the geology department of the University of 
Illinois, its source being unknown. The marcasite had a 
needle-like crystalline structure with dull grayish tin color, 
and showed slight oxidation on the surface. The minerals 
were identified by Penfield’s method® of distinction and 
shown to be correctly designated. The minerals were pulver- 
ized to the same degree of fineness and mixed in the propor- 
tion of 3 per cent of minerals with Randolph County coal and 
oxidized with saturated oxygen at two different tempera- 
tures—100° and 25° C. The difference between the sulfate 
content of the coal-pyrites mixture and that of the Randolph 
County coal when oxidized alone gives the rate of oxidation 
of the minerals. The results are graphically represented in 
Figures 5 and 6. 

Marcasite has a slightly higher rate of oxidation at 25° C., 
while at 100° C. the reverse is true for the sample ground to 
pass 325 mesh. This may be due to the fact that the in- 
crease of temperature has a slightly greater effect on the 
oxidation of pyrite than of marcasite. For the samples 
ground to pass through 140- and caught on 200-mesh sieve, 
the two curves are in general agreement when they were 
oxidized at 100° C., the pyrite having slightly higher rate 
for the first part of the experiment, while later the condition 
is reversed. 

This slightly higher rate of oxidation of marcasite at 25° C. 
does not necessarily indicate that it is more readily oxidized 
than pyrite. It is believed to be due more to its highly 
fragile nature, and to more fine material being produced 
during grinding. In samples of pyrite and marcasite ground 
to pass through a.200-mesh sieve, it was found that the 
iatter contains a higher proportion of the finer particles; 92 
per cent of marcasite passed through a 325-mesh sieve com- 
pared with 71 per cent of the pyrite. This ratio is probably 
also true in that proportion which passes through the 325- 
mesh sieve. 


Factor of Fineness of Division 


The effect of fineness of division was studied by separating 
the same sample of pyrite used in the previous experiment 
into four portions, mixing with Randolph County coal, and 
oxidizing each in saturated oxygen. 

From an examination of the results (Figure 7) several 
features are noticeable. The rate of oxidation seems to be 
a function of the size of particles, the oxidation of pyrite 
being nearly a linear function of its surface area. 

6 Brush and Penfield, ‘‘Descriptive Mineralogy,” 15th ed., 1898, p. 252. 


(7) 


We may assume that the surface areas of two powders of 
certain unit volume are in inverse ratio to the average diam- 
eter of their grains, as suggested by Purdy,’ if we assume 
first that the distribution of each portion is graded uniformly. 
It will be noted that the ratio of the reciprocal of the average 
diameters of the different portion and that of the percentage 
oxidation of the pyrite for 6 weeks are quite close. The 
relationship between the rate of oxidation, temperature, and 
the size of the particle can be expressed mathematically as 
follows: 


te 
R D K 
where R = rate of oxidation of pyrites 
T = temperature in ° C. 
D = diameter of the particles 
K =a constant; depends upon form of pyrite considered — 


This equation indicates in a general way that the rate of oxi- 
dation is directly proportional to the temperature and in- 
versely proportional to the diameter of the particles. 


Probable Catalytic Oxidation 


We have learned from the above experiments the nature 
or behavior of the pyrites alone when present in coal. How- 
ever, the presence of certain substances or. catalytic agents 
might affect the rate of oxidation of pyrite to a great extent. 
The following substances have been studied for their possible 
. catalytic action on the oxidation of pyrites by mixing them 
with Jackson County coal and oxidizing the mixture in 
saturated oxygen at room temperature: (1) Indiana clay, 
found underneath the kaolin deposit in Lawrence County, 
Indiana; (2) Tennessee ball clay; (3) coal highly saturated 
with oxygen; (4) sulfur oxidizing bacteria; (5) carbonated 
calcite water and common salt; and (6) mother of coal. 

The first three substances showed considerable catalytic 
action while the last three gave negative results. A word 
might be said about the Indiana clay. 

Logan® has described some experiments with black clay 
which was found underneath the kaolin deposit in Lawrence 
County, Indiana. This clay was shown to contain sulfur 
bacteria of some species closely related to that of the genus 
Beggiatoa. ‘To this type of organism was ascribed the mineral 
alteration from feldspar to pure kaolin. Some of this clay 
was obtained from the Gardner mine in order to test its 
possible action toward pyrites. The clay as received was 
yellowish rather than black, had fine texture, some plasticity, 
and granular structure. ; 

The so-called sulfur-oxidizing bacteria were a pure culture, 
Thiobacillis thio-oxidans, obtained from Dr. Waksman of the 
New Jersey Agricultural Experiment Station. The bacteria 
culture was kept in a liquid mixture® containing some sulfates 


1 Trans. Am. Ceram. Soc., 7, Pt. III, 441 (1905). See also Cushman 
and Hubbard, J. Am. Chem. Soc., 29, 589 (1907). 

8 Dept. of Conservation, State of Indiana, Publication 6, (1919). 

® Waksman and Joffe, Soil Sci., 12, 475 (1922). 


(8) 


and elementary sulfur. This culture was supposed to oxi- 
dize mainly elementary sulfur. Thus so long as the mixture 
contains any elementary sulfur, it would have no effect upon 
the oxidation of pyrite. The curves in Figure 8 show the 
result of this series of tests. 

Indiana clay has the greatest effect among the substances 
studied. In the presence of this clay the rate of oxidation of 
the sulfur in coal was about doubled. The catalytic action 
of Indiana clay was also tried on Vermilion County coal. 
The part of the total sulfur in this coal oxidized in 1 week 
was 4.19 per cent and 8.15 per cent in 2 weeks, while in the 
presence of the clay they were increased to 8.06 and 12.53 
per cent, respectively. The latter rate is practically equal 
to that at 100° C. 


i een 


a 


arcusife al\/00% 


Per Cent of Sulphur Oxidized 


| 
pars 


Bes 
ree 
isto: 


-sipid in nie 


BEE 


Figure 5—Comparison of Rate of Oxidation of Pyrite and 
Marcasite at 100° and 25° C. in Oxygen Saturated with 
Moisture, Both Minerals Passed through 325-Mesh Sieve 


Effect of Moisture and Oxygen Concentration 


In the previous experiments pure oxygen was used for 
oxidation for the purpose of hastening the reaction and thus 
reducing the time of the laboratory processes. Some experi- 
ments seem to be necessary to show the significance of con- 
centration of oxygen and its mode of action, as well as that 
of moisture in the coal. Six series of experiments were 
carried out in this direction on Vermilion County coal. The 
main features of the work may be outlined as follows: 


(9) 


(a) Coal containing high textural moisture was oxidized with 
oxygen saturated with moisture. 

(6) Coal containing high textural moisture was oxidized with 
air saturated with moisture. 

(c) Coal containing high textural moisture was oxidized with 
laboratory air unsaturated with moisture. 

(d) Coal was dried in the laboratory air until its moisture 
equilibrium was established and then oxidized with oxygen 
saturated with moisture. 

(e) Coal was dried as in (d) but was again sprinkled with 
water on the surface by means of an atomizer and then oxidized 
with oxygen saturated with moisture. 

(f) Coal containing its original textural moisture was sub- 
jected to slight pressure (about 15 inches of water) under oxygen 
for one week with the idea of letting the coal absorb as much 
oxygen as possible. Then the samples were stoppered in bottles 
and set aside. After each week determination of sulfate content 
was made on duplicate samples. 


The results of this series of tests are represented in Figure 
9. It will be noted that when air is used instead of oxygen 


3 


a 


| 


o 


> 


fs 


Per Cent of Sulohur Oxidized 
o 


Time in Weeks 


Figure 6—Comparison of Rate of Oxidation of Pyrite and 
“Marcasite at 100° and 25° C. in Oxygen Saturated with 
Moisture, Both Minerals Passed through 140-Mesh and 
Caught on 80-Mesh Sieve 


the rate of oxidation is reduced to about half of that for 
corresponding values with oxygen. In series (c), where the 
air was not saturated with moisture, the.amount of which 
depends on the relative humidity being fairly low in the 
laboratory atmosphere, the oxidation of pyrite was slow or 
practically negligible after the first week. The sulfate 
formed during the first week was derived from the moisture 
originally present in the coal. 

Series (d), (e), and (a) demonstrate clearly the effect of 
the moisture content of the coal on its textural condition. 
The removal of initial moisture from the coal decreases the 
rate of oxidation of sulfur, especially at the first stage. As 
the oxidation goes on, when enough moisture is absorbed 
into the coal the normal rate gradually recovers. Rewetting 
after drying does not restore the original speed quickly, the 
difference being probably due to uneven distribution of the 
moisture. After several weeks the moisture is more uniformly 


(10) 


distributed, and the oxidation goes on with its original 
rapidity or the rate may even be higher. 

In series (f) we provide another set of conditions for the 
oxidation of the sulfur. All the initial moisture is retained, 
plenty of oxygen is absorbed into the coal mass, and in addi- 
tion there are a few cubic centimeters of oxygen above the 
surface of the coal in the bottle. Everything that is neces- 
sary for the oxidation is present in a confined space; yet the 
oxidation goes on with extreme slowness. During the week 
when the samples were under oxygen pressure, there was only 
2.39 per cent of sulfur oxidized as compared with 4.19 per 
cent when it was oxidized in a moving stream of saturated 
oxygen at the same temperature. Therefore, the best con- 
dition for the pyrites to oxidize is in a moving stream of 
oxygen saturated with moisture. High moisture in coal 
seems always to be accompanied by rapid oxidation of the 
pyrites in the coal. As the moisture in the coal evaporates 
oxygen moves in to fill its place. The oxygen in this stage 
behaves in a way similar to oxygen in the nascent state, and 
appears to be much more active than the ordinary form of 
the gas. 


Discussion of Results 


As indicated at the beginning of this paper, the present 
study aims primarily to determine the ,oxidation behavior 
of pyrites at normal temperatures. There is ample evidence 
to show that at higher temperatures—say 75° C.—oxidation 
becomes so active as to proceed rapidly to the autogenous 
stage. The real question involved, therefore, is—how does 
coal reach those temperatures above the normal in coal 
piles aside from external sources such as hot walls, hot 
pipes, ete., which may bring the mass up to the danger 
point? The studies here given are directly concerned with 
the role that pyritic sulfur may play in this direction. It 
has been demonstrated that pyrites will oxidize when oxygen 
and moisture are present. The oxidation is quite rapid 
under suitable conditions and enough heat may be liberated 
to raise the coal to the danger point. 

The heat production of the oxidation of pyrites has been 
studied by Parr and Kressman,!° Winmill,? and Graham.’ 
The reaction may generally be represented by the following 
equation: 

2FeS, + 702 + 2H2O0 = 2FeSO, + 2H2SO. + heat 


The further formation of ferric sulfate is generally believed 
improbable due to the reducing action of the carbonaceous 
materials present in the coal. Parr and Kressman calcu- 
lated the heat produced indirectly from the heat of com- 
bustion of pyrites into ferric oxide and sulfur dioxide as. 
determined by Sommermeier. They found that for every 
two molecules of pyrites oxidized there are 624 large calories 
of heat liberated, not taking into account the heat of solution 
10 University of Illinois, Eng. Exp. Sta., Bull. 46 (1910). 


(11) 


of sulfuric acid or of the hydration of ferrous sulfate. This 
when figured to volume of oxygen absorbed is equivalent to 
4.1 calories for every cubic centimeter of oxygen absrobed. 
This calculation was confirmed by Winmill, who found by 
actual determination, 4.3 calories evolved for every cubic 
centimeter of oxygen absorbed. 

Let us now take one instance from this investigation and 
figure out its thermal effect, approximately as nearly as pos- 
sible the actual conditions on the coal piles. Vermilion 
County coal contains 3.75 per cent of sulfur and we may 
assume an oxidized condition of about 20 per cent. Suppose 


Per Cent of Sulphur Oxidized 


AY 
| mi 
ae 


D [ 
a a 

a ey: 
ae 
ui ge 


TU on 
|_| 
325) 
cab hog 
== 
80) on 4 


Figure 7—Oxidation of Pyrite at 100° and 25° C. in Oxygen 
Saturated with Moisture 
that the coal pile contains a layer of fines which in turn 
contains about 50 per cent of finer stuff of the same degree 
of fineness as the samples used for the experiments. ‘The 
dilution would probably decrease the heat effect on the 
temperature of the coal also about 50 per cent. Take 1 kg. 
of the coal into consideration; this amount of coal contains 
37.5 grams of sulfur. At the end of 6 weeks 20 per cent, or 
.75 grams, of it is oxidized. This amount of oxidation will 
evolve 37.6 large calories of heat. The specific heat of coal 
was first roughly determined by Trefall and more recently 


(12) 


by Coles. It was found to vary with the moisture content 
and theC: Hratio. In general it lies between 0.2 to 0.4, and 
the average value of 0.3 may be taken for illustration here. 
The amount of heat as obtained above will raise 1 kg. of coal 


37.6 
(Sn x 0. 5) 125.3°C. Allowing 50 per cent due to the dilu- 


tion by the coarser particles, which is assumed to be inert, 
we have a rise of 62.6°C. This will raise the temperature of 
the coal pile from 25° C. to above 85° C., which is well on the 
way to the danger point. 

This illustration is based on the natural rate of oxidation, 
independent of any of the catalytic agents. Should there be 
some catalytic agent present, such as Indiana clay, the rate 
can be at least doubled and vigorous heating is sure to occur. 
Furthermore, this estimation is based on the rate of oxida- 
tion at 25° C., but in effect the rate is always increasing 


a 


= 


hur Ox1d (zed 


ie eae 
Be 
omy 
fie 
el 
a 
a 
bala 
ii 
Ate 


/P 


\ 


SN eae 


i) 


Reet Ht+-HHe 


~ 


Per Cenk of Suf 
ieay 


GRaNNGaS 


fo 


REN S@i ae 
SONG er 
AN ees 


ZT aoe ahah 
Time in AE 


Figure 8—Oxidation of Sulfurin Jackson County Coal at 25 °C 
in Presence of Various Catalytic Agents 


LG 


° -~ 


gradually due to the augmentation of heat given out from 
the reaction, if it is not dissipated. If we take this into 
consideration, the estimation would be too low. ‘The accel- 
erating effect of increased temperature might more than 
offset radiation losses. 

Coals may also contain a pocket or lens of pyrites in a 
finely divided condition. This would certainly make a hot 
spot in the coal pile and its ignition action would be probable. 
Of course the behavior of pyrites in coals differs widely, as 
shown by the first part of this investigation. Some oxidize 
very rapidly and others are much more inert. Some coals 
have low sulfur content and the rate of oxidation of the 


(13) 


latter may also be slow. In such a case the self-heating, 
should it happen in the coal, must be traced to the oxidation 
of carbonaceous material. On the other hand, in high- 
sulfur coal the pyrites may be a contributing cause for heat 
generation. In certain cases, indeed, pyrites may be the 
chief cause while in other cases the carbonaceous material 
may be active. Should they both be operative in the same 
coal they may each contribute to the final heating. Pre- 
vious reports may sometimes have been mistaken in stating 
that pyrites could not be the cause of spontaneous combus- 
tion of coal because some coals had ignited with very low 
sulfur content; others which say that the heat resulting 
from the oxidation of pyrites is not enough to raise the tem- 


(ay led aa ae we ee 
fy Z| nb 
eat sohe oA 


Johure fee 


Ka 


Per Cent of Sulphur Oxidized 


BS 
by 


oi EEE: 


El = 
ECON 


Ss 
mm 
> 
oN ~ 
~ 
Uy 
ny 


iW, ie E 
LH) s2ep sen) 
A eee 
o S 

Ti cP in Weeks 


Figure 9—Oxidation of Sulfur in Vermilion County Coal at 
25° C. with Varying Conditions of Oxygen and Moisture 


perature of the coal to ignition may also be in error. Coal 
is a very complicated substance and its characteristics vary 
widely according to its origin and types. Many factors 
enter into the heating of coal; each may contribute a certain 
part and codperate with the others in the development of 
initial heat. 


Summary and Conclusions 


1—Under suitable conditions the pyrites in coal a 
oxidize rapidly and may be a dominating factor in certain 
cases for the self-heating of coal. 


(14) 


2—Marcasite and pyrite oxidize with about the same rate, 
but the former breaks down more easily producing fine 
particles, thus facilitating its oxidation. 

3—The rate of oxidation of pyrite or marcasite is directly 
proportional to temperature and inversely proportional to 
the diameter of the particles. | ? 

4—Dry air or oxygen does not promote the oxidation of the 
pyrites. 

5—High textural moisture seems to be accompanied by 
rapid oxidation of the pyrites. A moving stream of oxygen 
saturated with moisture seems also to be the best accompani- 
ment for the oxidation of pyrites in the presence of high 
textural moisture. The fact that high textural moisture 
promotes oxidation of pyrite is presumably due to the evapo- 
ration of the moisture and its subsequent replacement by 
oxygen. The oxygen in this state appears to be much more 
active than under ordinary conditions. ‘There may be a 
catalyzing effect set up by this interchange of moisture and 
oxygen or a possible activation of the coal surfaces which 
results in a greater activity being imparted to the oxygen. 

6—The catalytic oxidation of certain substances such as 
Indiana clay is presumably due to the presence of a certain 
type of bacteria in the clay. That the catalytic action is 
not from the colloidal nature of the clay alone seems evident 
from a comparison of the experiment with ball clay, which 
probably contains more of the colloidal material, yet does not 
produce so much catalytic action. 


(15) 


cd 


Wate’ 


The writer of this thesis was born in Vhing, 
Kiangsu, China, February 1, 1898. Following his 
primary school training, he entered the Fifth 
Provincial Middle School at Chang Chow, Kiangsu, 
China, and graduated after a four years course in 
1917. He entered the University of. Nanking, at 
Nanking, China, in the fall of 1917, registering in 
the College of Agriculture. After three years at 
Nanking, he came to the United States and entered 
the University of Michigan, registering in the 
course of chemical engineering. He was at Michigan 
for one semester and then transferred to the Uni- 
versity of Illinois, and graduated from the course 
in chemical engineering in 1922. In the fall of 1922 
he took up graduate work in the department of 
Chemistry of the University of Illinois and has 
continued in the work until the present time. He 
received. the degree of M. 8S. in Chemistry in 1923. 


While in attendance at the University of Illinois 
he has held the position of Graduate Research As- 
sistant in Chemical Engineering from 19238 to date. 


3 0112 073236934 


coe ‘ : 


